It is well known in the cigarette filter art to provide air ventilation or air dilution means for introducing ambient air into the filter to dilute the mainstream smoke passing from the tobacco rod through the filter. As used herein, "air dilution" or "air ventilation" refers to ambient air as a diluent, and is the ratio usually expressed as a percentage, of the volume of air drawn through the air dilution means to the total volume of air and aerosol drawn through the cigarette and exiting the mouth end of the cigarette. Dilution of the mainstream smoke reduces the quantity of particulate matter and gas phase components of the smoke that is delivered to the smoker.
Ventilating air has heretofore been introduced into the filter in several ways, but primarily by the use of porous or perforated wrappers for the tobacco rod and/or filter. The most common means for introducing ventilating air into a cigarette has been by means of mechanical or laser perforations of the tipping paper and the filter plug wrap. Typically, a plurality of perforations or openings are provided in one or more rows arranged circumferentially about the filter adjacent the tobacco rod end thereof. The openings may be provided in the tipping paper and plug wrap prior to or during assembly of the cigarette.
Cigarette filters may be made with many different types of filtration materials, one of the most common materials being a rod of fibrous material, such as a cellulose acetate fiber tow. When a smoker draws on the mouth end of a cigarette with a conventional air diluted cellulose acetate filter, air flows through the air dilution openings in the tipping paper and plug wrap in a direction generally transversely to the longitudinal axis of the filter until it meets the flow of mainstream smoke from the tobacco rod. The diluting air flow is diverted by the pressure drop across the filter in a longitudinal direction toward the mouth end of the filter. In this conventional air diluted filter construction, the mainstream smoke flow is usually concentrated in a central or core portion of the filter and the dilution air flows primarily in the annular portion of the filter surrounding the central portion and between such central portion and the filter plug wrap.
Such flow of mainstream smoke and diluting air is evident from a visual inspection of the staining pattern at the mouth end of the filter. Since the dilution air forces the mainstream smoke toward the longitudinal axis of the filter, the "tar" or particulate matter in the smoke will be concentrated in the central or core portion of the filter leaving a stained or discolored core portion and an annular or peripheral portion of the filter unstained or substantially unstained. In the case of a conventional cellulose acetate filter, this flow pattern results in a concentrated stain or discoloration in the core portion of the mouth end of the filter surrounded by a white or substantially white peripheral region which does not discolor significantly as the cigarette is smoked. The greater the percentage of air dilution, the lesser the staining or discoloration in the core portion and the greater the peripheral unstained or white peripheral region.
Many factors affect the flavor and taste of cigarette smoke including filtration and air dilution. An increase in filtration efficiency (i.e., an increase in pressure drop across the filter element) or an increase in air dilution ordinarily will reduce the flavor perceived by the smoker. It would be desirable, however, to provide a filter for a cigarette which has sufficient air dilution to significantly reduce the particulate matter and gas phase components delivered to the smoker while retaining the taste and flavor of the smoke to the greatest extent possible.